The Integumentary System: Structure and Function

The Integumentary System

Overview of the Skin (Integument)

The skin, or integument, is the largest organ of the human body and serves as a protective barrier. It consists of two main regions and an underlying layer:

• Epidermis: Superficial region composed of epithelial tissue.

• Dermis: Deeper region made of connective tissue.

• Hypodermis (superficial fascia): Subcutaneous layer deep to the skin, primarily composed of adipose tissue. It anchors the skin to underlying structures, mainly muscles. Additional info: The hypodermis is not technically part of the skin but is important for fat storage and insulation.

Structure of the Skin

Epidermis

The epidermis is a keratinized stratified squamous epithelium that forms the outermost protective shield of the body. It is avascular and consists of four or five distinct layers, depending on the region of the body.

• Keratinocytes: Produce keratin, a fibrous protein that gives the epidermis its protective properties.

• Melanocytes: Synthesize melanin, the pigment responsible for skin color.

• Dendritic (Langerhans) cells: Macrophages that help activate the immune system.

• Tactile (Merkel) cells: Sensory receptors for touch.

Layers of the Epidermis

• Stratum Basale (Basal Layer): Deepest layer, single row of stem cells, site of mitosis. Contains melanocytes (10-25% of cells).

• Stratum Spinosum (Prickly Layer): Several layers thick, contains intermediate filaments and desmosomes, abundant dendritic cells.

• Stratum Granulosum (Granular Layer): Thin layer, cells flatten, keratinization begins, organelles disintegrate, cells above this layer die due to lack of nutrients.

• Stratum Lucidum (Clear Layer): Only in thick skin (palms, soles), a few rows of flat, dead keratinocytes.

• Stratum Corneum (Horny Layer): 20-30 layers of dead, flat, anucleate keratinized cells, provides protection against abrasion and penetration, waterproofs the skin.

Cell Differentiation in the Epidermis

• Cells migrate from the stratum basale to the stratum corneum, undergoing specialized apoptosis (programmed cell death).

• Dead cells are shed from the surface; approximately 50,000 cells are lost every minute.

Dermis

The dermis is a strong, flexible connective tissue layer beneath the epidermis. It contains fibroblasts, macrophages, and occasionally mast cells and white blood cells. The dermis is richly supplied with nerve fibers, blood vessels, and lymphatic vessels, and contains hair follicles, oil, and sweat glands.

Layers of the Dermis

• Papillary Layer: Areolar connective tissue with collagen and elastic fibers, contains dermal papillae (projections) that house capillary loops, touch receptors, and sometimes free nerve endings. Forms friction ridges in thick skin (fingerprints).

• Reticular Layer: ~80% of dermal thickness, dense irregular connective tissue, provides strength and resiliency, contains cleavage lines (important for surgical incisions) and flexure lines (dermal folds at joints).

Skin Markings

• Friction ridges: Enhance gripping ability, contribute to fingerprints.

• Cleavage (tension) lines: Indicate the direction of collagen fibers, important for surgical incisions.

• Flexure lines: Dermal folds at or near joints.

• Striae: Stretch marks, result from dermal tearing.

• Blisters: Fluid-filled pockets that separate epidermal and dermal layers.

Skin Color

Skin color is determined by three main pigments:

• Melanin: Ranges from yellow to brown to black, produced by melanocytes, protects against UV radiation.

• Carotene: Yellow to orange pigment, most obvious in palms and soles, can be converted to vitamin A.

• Hemoglobin: Red pigment in red blood cells, gives a pinkish hue to fair skin.

Clinical Relevance of Skin Color

• Cyanosis: Blue skin color due to low oxygenation.

• Erythema: Redness due to fever, hypertension, inflammation, or allergy.

• Pallor: Pale color due to anemia, low blood pressure, or fear.

• Jaundice: Yellow cast due to liver disorders.

• Bronzing: Metallic appearance, may indicate Addison's disease.

• Bruises: Black-and-blue marks due to clotted blood beneath the skin.

Appendages of the Skin

Skin appendages are derivatives of the epidermis and include hair, hair follicles, nails, sweat glands, and sebaceous (oil) glands.

Hair

• Consists of dead keratinized cells.

• Functions: Sensation, protection, and insulation.

• Regions: Shaft (above skin), root (within skin), and bulb (deepest part).

• Types: Vellus hair (fine, body hair of children and adult females), Terminal hair (coarse, long hair of scalp, eyebrows, axillary, and pubic regions).

• Growth: Average 2.25 mm per week; growth cycles alternate between active and regressive phases.

• Alopecia: Hair thinning or baldness, can be genetic or due to hormonal changes.

Hair Follicles

• Extend from the epidermal surface into the dermis.

• Structure: Two-layered wall (outer connective tissue root sheath and inner epithelial root sheath).

• Associated structures: Hair bulb, hair matrix (growth zone), arrector pili muscle (causes hair to stand), sebaceous glands.

Nails

• Scale-like modifications of the epidermis.

• Protective covering for the distal, dorsal surface of fingers and toes.

• Contain hard keratin.

• Nail matrix is responsible for nail growth.

Sweat Glands (Sudoriferous Glands)

• Distributed over most of the skin surface.

• Two main types: Eccrine and Apocrine glands.

Eccrine Sweat Glands

• Most numerous, abundant on palms, soles, and forehead.

• Ducts connect to pores on the skin surface.

• Function: Thermoregulation via sweat secretion (99% water, salts, vitamin C, antibodies, dermcidin, metabolic wastes).

• Regulated by the sympathetic nervous system.

Apocrine Sweat Glands

• Confined to axillary and anogenital areas.

• Ducts empty into hair follicles.

• Secrete viscous, milky or yellowish sweat containing fatty substances and proteins.

• Begin functioning at puberty; may act as sexual scent glands.

• Specialized types: Ceruminous glands (earwax) and Mammary glands (milk).

Sebaceous (Oil) Glands

• Widely distributed, except in thick skin of palms and soles.

• Most develop from hair follicles and secrete into hair follicles.

• Secrete sebum: Oily holocrine secretion that softens hair and skin, has bactericidal properties.

Summary Table: Layers of the Epidermis

Layer	Main Features	Location
Stratum Corneum	20-30 layers of dead, keratinized cells; protection	Outermost
Stratum Lucidum	Few rows of flat, dead keratinocytes	Only in thick skin (palms, soles)
Stratum Granulosum	Keratinization begins; organelles disintegrate	Middle
Stratum Spinosum	Several layers; desmosomes; dendritic cells	Above basal layer
Stratum Basale	Single row of stem cells; mitosis; melanocytes	Deepest

Key Equations and Concepts

• Melanin Synthesis: Tyrosinase catalyzes the conversion of tyrosine to melanin in melanocytes.

• Water Loss through Skin: $\text{Insensible perspiration} \approx 500 \text{ mL/day}$

• Thermoregulation: $\text{Heat loss} = \text{Sweat evaporation} \times \text{Latent heat of vaporization}$

Additional info: The integumentary system also plays roles in vitamin D synthesis, excretion, and sensory reception.